High fidelity sound reproducer



Aug. 3, 1965 s. R. RICH HIGH FIDELITY SOUND REPRoDucER Filed June 14.1961 5 m. R m H N m @L M c 4 A. I R @if G. m N F n 2 8 En s G. al/ H m/Tu/ 5 m//v L. u m\\1 w I e\\| 1 2 r) IM x d n 5f invii n.\ 4 n 1. e/ 2 [1l 5I\\T L 2 5 yAT TORNEY United States Patent O 3,198,899 HlfGH FIDELTYSOUND REPRODUCER Stanley 12. Rich, West Hartford, Conn., assigner, bymestre assignments, to Alfred H. Rosen, Newton, Mass. Filed .lune 14,1961, Ser. No. 117,0110 10 Claims. (Cl. 179-4155) This invention relatesto electromechanical transducer devices for reproducing sound with highfidelity, and more particularly to loudspeakers for use in the highfrequency portion of the audible sound spectrum.

It is an object of this invention to provide an improved loudspeaker forthe upper audible frequencies. Another object is to provide such aloudspeaker which faithfully reproduces sounds without contributingdistortions., or resonances, of its own. A further object is to providesuch a loudspeaker in a structure which is compact, simple to construct,requires no unusual parts, and is readily adapted to cooperate withother loudspeaker units in a complete loudspeaker system.

According to the present invention, a diaphragm made of a sheet ofelectrically conductive material disposed in a iiattened magnetic fieldhaving lines of force generally parallel to the surfaces of the sheet,and a preferably flat coil of Wire, serving as a voice coil, is disposedparallel to and sufficiently close to the diaphragm to induce eddycurrents into the conductive material thereof from currents traversingthe voice coil. The voice coil is fixed on a platform which is ofsubstantial mass compared to the diaphragm, and the diaphragm is highlycompliant, of

light weight, and, Within prescribed limits, free to move in eitherdirection normal to the magnetic field. The platform may be part of apermanent magnet structure, and the space enclosed between the platformsurface and the diaphragm, containing the voice coil, may behermetically sealed.

An eddy current-type sound reproducer is shown in U.S. Patent No.1,577,254 granted March 16, 1926 to Hayes. ln this reproducer themagnetic field is perpendicular to the surface of a stiff conicaldiaphragm, and the only communication between the diaphragm and alistener is through a hole in the apex of a conical gap in the magnetstructure. The hole is very small compared to the total area of thediaphragm, so that most of the energy available to make sound isdissipated between the magnet poles, as friction. Further, the eddycurrent driving the force at every point in the diaphragm is resolvedinto two components, one radial to the axis of the cone and the otherparallel to the cone axis, and only the latter component is useful togenerate sound waves. The Hayes device is, therefore, quite inefficient.

Another eddy current-type loudspeaker is shown in U.S. Patent No.1,643,791 granted September 27, 1927 to Slepian. a fiat voice coil,which is connected to a battery to produce a magnetic field. This field,like that of Hayes, has its lines of force normal to the diaphragm.Voice currents superposed upon the battery current are stated to causechanges in this field which induce eddy currents into the conductivesheet, and thereby to cause the sheet to be moved away from and towardthe coil. Gravity enters into the forces tending to move the diaphragmtoward the coil. It is not explained how the diaphragm is prevented fromrattling when it falls back on the coil, or how it is made to move in adirection parallel to the magnetic field.

The reproducer of the present invention differs drastically from theprior art structures. For example, the diaphragm moves in directionsnormal to the magnetic field; the field is non-varying and isconcentrated largely in the diaphragm, so that a most efficientcombination results. The diaphragm is free to move, within adequatelimits, in either direction without bumping into other Here a sheet ofconducting material rests on 3,198,890 Patented Aug. 3, 1965 ICC partsof the structure; it can be hydraulically as well as eddy-current dampedand presents its entire movable surface to a listener. Other and furtheradvantages and features of the invention will become apparent from thefollowing description of an embodiment thereof. This description refersto the accompanying drawings, wherein:

FIG. 1 is an isometric view of the magnetic field structure of thisembodiment of the invention, showing in dotted line the disposition of avoice coil thereon;

FlG. 2 is a section along line 2-2 in FIG. l, of a completely assembledloudspeaker;

PEG. 3 illustrates the magnetic field pattern;

FIG. 4 is a detail showing the construction of the voice coil; and FiG.5 is a front view of the transducer as it appears fully assembled.

Referring to FIG. 1, first and second permanent bar magnets 11 and 12,having poles on opposite long faces thereof, are disposed with likepoles., here north poles, confronting each other. A spacer bar 13, ofcold-rolled steel, for example, is disposed between the confrontingnorth poles. End plates 14 and 1S, also of cold-rolled steel, forexample, are disposed one at each south pole. These parts lare all heldtogether by a suitable cement, such as an epoxy cement. It is alsopossible to hold them together with one or more bolts (not shown)passing through the structure from one end plate 14 to the other 15; theend plates provide suitable bearing surfaces for such bolts. Obviously,each magnet 11 or 12 could be made of a series of smaller magnets, as isindicated at the dotted lines 11.5 and 12.5, respectively.

The spacer bar and end plates 13, 14 and 15 have edge portions 13.5,14.5 and 15.5 projecting above a platform 16 constituted by the surfaceat the top (in FIG. 1) of the magnets 11 and 12. The projecting edgeportion 13.5 of the spacer bar is shorter than the others, the endportions of the spacer bar 13 being flush with the platform 16. A voicecoil 17 (FIG. 2) omitted from FIG. 1 to permit a clear illustration ofthe magnetic field structure, occupies a position on platform 16 whichis indicated by a fiat spiral dotted line 17.5. Terminals for the voicecoil are indicated at' 18 and 19.

FIG. 2 shows the wires of the voice coil 17 in crosssection, restingdirectly on the platform 16. FIG. 4 shows a voice coil subassembly 21,which may be used to prefabricate the voice coil. The Wires of the voicecoil are cemented to a support 22 of paper, as with an epoxy cement. Afiat spiral coil of about 40 turns of No. 28 phenolic enamel insulatedcopper magnet Wire, all in one layer, has been used in an area about 2inches by 21/2-3 inches. The paper support 22 has an aperture 23 in it,so that it will slip over the edge projection 13.5 of the spacer bar 13.In an embodiment built according to the drawings, the dimensions of themagnet structure shown in FIG. 1 were about 3 x 3" x 3A, and the lengthand width dimensions of the voice coil support 22 were about 3 longparallel to the end plates 14 and 15 and about 21/2 wide between them.The voice coil 17 is mounted on the platform 16 by cementing thesubassembly Z1 thereto, using for example, an epoxy cement.

Referring vagain to FIG. 2, two diaphragm mount bars 24 and 25, whichmay be made of Wood, are cemented, respectively, one on each edge of theplatform 16 closely adjacent the edge projection 14.5 or 15.5 of theadjoining endY plate 14 or 15. A diaphragm 26 is cemented at its edgesto these mount bars, and at a strip along its center to the top edge ofthe edge projection 13.5 of the spacer bar 13. The diaphragm is made ofaluminum foil, 1/2 to 2 mil. inches thick, and is corrugated as. shown,to assure highcompliance to motion to and from the platform 16, With nodifficulty from stretching between the lineal re- 3 gions where it ismounted to the mount bars 24 and 25 and the spacer bar projection 13.5.An epoxy cement is useful to assemble the diaphragm, mount bars andmagnet structure.

Referring to FIG. 3, it will be seen that the edge projection 13.5 ofthe spacer bar 13 constitutes an elongated north magnetic pole and theedge projections 14.5 and 15.5 of the end plates 14 and 15,respectively, each constitute an elongated south magnetic pole. Dottedlines 28 represent the magnetic field configuration .between the northand each south pole. This is a eld which is symmetrical and essentiallyrectangular to both sides of the north pole, and the diaphragm 26 isdisposed essentially entirely within this field. The voice coil 17 isapproximately l/@ inch away from the diaphragm, the mount bars beingabout 1/16 inch thick, so that an electromagnetic field set up aroundthe voice coil due to voice modulated currents therein can induce eddycurrents into the aluminum diaphragm 26. The latter being in thestrongest part of the permanent magnetic field 28, these eddy currentswill react powerfully with the permanent magnetic field, causing mot-ionof the diaphragm perpendicular to the permanent field 28 fluctuatingaccording to the modulation, and faithful reproduction of sound.

FIG. 5 shows a completed loudspeaker in which a wood frame is provided,comprising members 31, 32, 33 and 34 at the edges of the magnetstructure, surrounding this entire structure. This frame facilitatesmounting the loudspeaker in a wood cabinet, for example. The diaphragm26 is shown covering the spacer bar projection 13.5 (shown in dottedline) and partially covering the mount bars 24 and 25, as illustrated inFIG. 2. The free edges of the end plate projections 14.5 and 15.5 can beseen exposed in FIG. 5.

The diaphragm 26 extends at its ends 26.1 and 26.2 to cover theadjoining frame members 31 and 33, respectively. In this Way thediaphragm hermetically seals the space between the diaphragm and theplatform 16. It will be understood that this space is entirelysurrounded by wooden members, namely the diaphragm mount bars 24 and 25and the frame members 31 and 33 perpendicular thereto, the jointsbetween which are all sealed with cement, such as an epoxy cement. Therear surface of the diaphragm is thus provided with a sealed chamber.The aluminum foil of the diaphragm 26 is so thin that the corrugations,as seen in FG. 2, can easily be made Without interfering with theability to preserve suicient smoothness of the ends 26.1 and 26.2 forthe purpose of cementing them down. Alternatively, the corrugations canbe carried to the ends and lled with cement; they f are very small-lessthan 1/16 inch deep, being exaggerated in FIG. 2 for the purpose ofillustration.

A loudspeaker built as described above, using two magnets on each sideof the spacer bar 13, each about 3 long and 5/8 square, is an excellentand most efficient reproducer in the frequency range from 5,000 to30,000 c.p.s. The exciting coil 17 being an inductor, the reactancefalls off at lower frequencies. A loudspeaker employing the sameprinciples can, however, be designed for use in a lower frequency range.

With the diaphragm 26 sealed at all of its edges, and provided at itsrear surface with a sealed chamber, as in FIG. 5, only the front surfaceis acoustically exposed to a listener, so that no radiation is presentfrom the back surface, and distortion from that source does not exist.in addition, the sealed back chamber provides the diaphragm with thehydraulic damping, in addition to the eddy current damping which is tobe expected from the presence of the aluminum sheet in a magnetic field.Furthermore, the sealed back chamber can be used to move the resonanceof the diaphragm 26 to a frequency range higher than audible.

The embodiments of the invention which have been illustrated anddescribed herein are but a few illustrations of the invention. Otherembodiments and modifications will occur to those skilled in the art. Noattempt has been made .to illustrate all possible embodiments of theinvention, but rather only to illustrate its principles and the bestmanner presently known to practice it. rfherefore, while certainspecific embodiments have been described as illustrative of theinvention, such other forms as would occur to one skilled in this art ona reading of the foregoing specification are also within the spirit andscope of the invention, and it is intended that this invention includesall modifications and equivalents which fall within the scope of theappended claims.

What is claimed is:

1. Loudspeaker comprising an electrically conductive diaphragm, meansproviding a magnetic field having lines of force directed substantiallyparallel to said diaphragm, said diaphragm being disposed within saidfield, voice coil means disposed adjacent one side only 4of saiddiaphragm adapted to induce eddy currents therein in responce tomodulated currents owing in said voice coil means, said voice coil meansbeing spaced from said diaphragm so that Within prescribed limits saiddiaphragm may move toward and away from said voice coil means withoutcontacting the same, and means including said diaphragm as a Wall memberthereof for providing a chamber at said one side of said diaphragm.

2. Loudspeaker comprising an electrically conductive diaphragm, meansproviding a magnetic field having lines of force directed substantiallyparallel to said diaphragm, said diaphragm being disposed within saidfield, a flat spiral voice coil parallel and adjacent one side only ofsaid diaphragm, said voice coil means being spaced from said diaphragm adistance such that, within prescribed limits, said diaphragm may movetoward and away from said voice coil without contacting the same, andsimultaneously such that modulated currents flowing in said voice coilwill induce eddy currents in said diaphragm, and means including saiddiaphragm as a wall member thereof for providing a chamber at said oneside of said diaphragm.

3. Loudspeaker comprising an electrically conductive diaphragm, meansproviding a magnetic field having lines of force directed substantiallyparallel to said diaphragm, said diaphragm being disposed within saidfield, a platform parallel to and spaced a distance from said diaphragmconfronting one side of said diaphragm, a fiat spiral voice coil fixedlymounted on said platform, means supporting said diaphragm relative tosaid platform, said distance being such that, within prescribed limits,said diaphragm may move toward and away from said voice coil withoutcontacting the same, and simultaneously such that modulated currentsowing in said voice coil will induce eddy currents in said diaphragm,and means including said diaphragm as a wall member thereof forproviding a chamber at said one side of said diaphragm.

4. Loudspeaker comprising an electrically conductive diaphragm, apermanent magnet having magnetically permeable members respectively oneat each pole, said members having projections extending beyond a side ofsaid magnet, means mounting said diaphragm to said magnet between saidprojections, said diaphragm being thereby disposed parallel to said sidewith one side of said diaphragm confronting the same and spaced adistance therefrom, voice coil means fixedly mounted on said side ofsaid magnet, said distance being such that, within prescribed limits,said diaphragm may move toward and away from said voice coil withoutcontacting the same, and simultaneously such that modulated currentsowing in said voice coil will induce eddy currents in said diaphragm,and means including said diaphragm and said magnet as wall membersthereof for providing a chamber at said one side of said diaphragmenclosing said voice coil.

5. Loudspeaker comprising an electrically conductive diaphragm, firstand second elongated permanent magnets of rectangular cross section,each magnet hav-ing a north pole at one long face and a south pole atthe opposite long face, a flat spacer bar of magnetically permeablematerial, said magnets disposed with one pair of like poles confrontingand said spacer bar vbetween them, irst and second magneticallypermeable end plates disposed respectively one at each of the remainingpair of like poles, side surfaces of said magnets between said poles ofeach lying in a common plane, said spacer bar and end plates having edgeportions projecting beyond said plane, said diaphragm being supported atits edges between said edge portions of said end plates and at anintermediate region from said edge portion of said spacer bar, and a atspiral voice coil fixedly mounted on said side surfaces of said magnetsconfronting a surface of said diaphragm, the distance between saiddiaphragm and said voice coil being such that, within prescribed limits,said diaphragm may move toward and away from said voice coil withoutcontacting the same, and simultaneously such that modulated currentsowing in said voice coil will induce eddy currents in said diaphragm.

6. Loudspeaker comprising an electrically conductive diaphragm, meansproviding a magnetic field having lines of force directed substantiallyparallel to said diaphragm, said diaphragm being disposed within saidheld, a at platform parallel to and spaced from said diaphragm voicecoil means ixedly mounted in said platform and disposed adjacent andconfronting one side only of said diaphragm adapted to induce eddycurrents therein in response to modulated currents flowing in said voicecoil means, said voice coil means and platfrom being spaced `rom saiddiaphragm so that within prescribed limits said diaphragm may movetoward and away from said voice coil means Without contacting the same,and means including said diaphragm and the mounting means for said voicecoil means for providing an acoustically sealed chamber around said sideof said diaphragm.

7. Loudspeaker comprising an electrically conductive rectangulardiaphragm, means providing a magnetic eld having lines of force directedsubstantially parallel to said diaphragm, the direction of said lines offorce being symmetrical between two parallel edges of said diaphragm anda line in said diaphragm parallel to and substantially between said twoedges, said diaphragm being disposed within said field, a at rectangularspiral voice coil lying in a plane parallel and adjacent one side onlyof said diaphragm, said coil having an elongated rectangular openportion in the center in register with said line, said voice coil beingspaced from said diaphragm a distance such that, within prescribedlimits, said diaphragm may move toward and away from said voice coilwithout contacting the same, and simultaneously such that modulatedcurrents flowing in said voice coil will induce eddy currents in saiddiaphragm, and means including said diaphragm as a wall member thereoffor providing a chamber at said one side of said diaphragm.

S. Loudspeaker comprising an electrically conductive diaphragm, meansproviding a magnetic field having lines of force directed substantiallyparallel to said diaphrag said diaphragm being disposed Within said eld,a platform parallel to and spaced a distance from said diaphragmconfronting one side only of said diaphragm, a dat spiral voice coilxedly supported on said platfrom, means supporting said diaphragmrelative to said platform, said distance being such that, withinprescribed limits, said diaphragm may move toward and away from saidvoice coil without contacting the same, and simultaneously such thatmodulated currents flowing in said voice coil will induce eddy currentsin said diaphragm, and means including said diaphragm and said platformas Wall members thereof for providing substantially an acousticallysealed chamber at said one side of said diaphragm.

9. Loudspeaker comprising a substantially flat electrically conductivediaphragm, a permanent magnet having magnetically permeable membersrespectively one at each pole, said members having projections extendingbeyond a side of said magnet, means mounting said diaphragm to saidmagnet between said projections, said side being flat, a flat spiralvoice coil iixedly mounted on said side, said diaphragm being disposedparallel to said side with one side of said diaphragm confronting thesame and spaced a prescribed distance therefrom such that, withinprescribed limits, said diaphragm can move toward and away from saidvoice coil without contacting the same, and simultaneously such thatmodulating currents owing in said voice coil will induce eddy curents insaid diaphragm.

10. Loudspeaker comprising an electrically conductive diaphragm,elongated permanent magnet means of rectangular cross-section and havinga first pole at one long face and a second pole at the opposite longface, first and second magnetically permeable end plates disposedrespectively one at each of said poles, said end plates having edgeportions projecting beyond lone side of said magnet means, said sidelying in a plane between said long faces, said diaphragm being supportedfrom said edge portions substantially parallel to said plane, and a flatvoice coil xedly mounted on said one side of said magnet meansconfronting a surface of said diaphragm, the dis- `tance between saiddiaphragm and said voi coil being such that, within prescribed limits,said diaphragm may move toward and away from said voice coil withoutcontact-ing the same, and simultaneously such that modulating currentsflowing in said voice coil will induce eddy currents in said diaphragm.

References Cited bythe Examiner UNITED STATES PATENTS 1,23 8,439 8/17Rummler 179-1155 1,934,184 11/33 Gerlach et al. 179-1155 1,955,390 4/34Schifl 179-1155 2,971,597 2/61 Gorike 179-l15.5

FOREIGN PATENTS `574,276 12/ 23 France.

ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner.

1. LOUDSPEAKER COMPRISING AN ELECTRICALLY CONDUCTIVE DIAPHRAGM, MEANSPROVIDING A MAGNETIC FIELD HAVING LINES OF FORCE DIRECTED SUBSTANTIALLYPARALLEL TO SAID DIAPHRAGM, SAID DIAPHRAGM BEING DISPOSED WITHIN SAIDFIELD, VOICE COIL MEANS DISPOSED ADJACENT ONE SIDE ONLY OF SAIDDIAPHRAGM ADAPTED TO INDUCE EDDY CURRENTS THEREIN IN RESPONCE TOMODULATED CURRENTS FLOWING IN SAID VOICE COIL MEANS, SAID VOICE COILMEANS BEING SPACED FROM SAID DIAPHRAGM SO THAT WITHIN PRESCRIBED LIMITSSAID DIAPHRAGM MAY MOVE TOWARD AND AWAY FROM SAID VOICE COIL MEANSWITHOUT CONTACTING THE SAME, AND MEANS INCLUDING SAID DIAPHRAGM AS AWALL MEMBER THEREOF FOR PROVIDING A CHAMBER AT SAID ONE SIDE OF SAIDDIAPHRAGM.